Sorry if anyone has already said this/
But the enemies behind the wall before the bridge extension that you pull the lever to have "Crimson" in their name. They consist of a pack of the war hounds and a flamethrower guy. So they are most likely void as well (if we are going on the different elemental theory).
This would still leave solar out there though...
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Maybe only kill the flamer and run around with the dogs alive and see if the other door opens after a while? Dont remeber the layout of the mission exactly. But maybe lure the dogs into the drill with 1 person and have the other activate the drill. I know it doesnt activate if a player is inside,but it closes as soon as you exit it. Maybe trapping the dogs inside and triggering something.